Enhancement of syngas production in coal gasification with CO2 conversion under plasma conditions

ABSTRACT

A process and apparatus for enhancement of syngas production (CO and H 2 ) of a carbon based feedstock with CO 2  conversion, which utilized CO 2  as an oxygen resource and converts CO 2  to CO through chemical reactions. The process includes a thermal plasma reactor and optionally a nonthermal plasma reactor.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with United States Government support underContract No. HR0011-09-C-0094 awarded by the U.S. Defense AdvancedResearch Projects Agency. The Government has certain rights in thisinvention.

FIELD OF THE INVENTION

The present disclosure is directed at the enhancement of syngasproduction in coal gasification. More specifically, the presentdisclosure relates to a carbon dioxide reduction process which utilizescarbon dioxide as an oxygen source and converts carbon dioxide to carbonmonoxide in a plasma coal gasification process. The process utilizes athermal plasma reactor for syngas production and carbon dioxideconversion.

BACKGROUND

Coal and other fossil fuels will remain critical fuels for the U.S. andsome other countries. The development and deployment of lower CO₂emissions technologies will be a key in delivering clean energy andclimate control associated with fossil fuel use. In the recent years,global efforts in carbon dioxide (CO₂) capture and storage (CCS) aregaining importance in significant greenhouse gas emission reductionsfrom the use of fossil energy. Carbon dioxide capture-enablingtechnologies in coal-fired energy area include integrated coalgasification combined cycle (IGCC), oxy-fuel combustion, andpost-combustion CO₂ capture. These technologies are at different stagesof development.

SUMMARY

A process for carbon dioxide conversion comprising supplying a thermalplasma reactor and introducing into the reactor a carbon based feedstock(C) and carbon dioxide (CO₂) wherein the thermal plasma reactor is at atemperature sufficient to convert the carbon based feedstock to carbonmonoxide according to the following reaction:C+CO₂⇄2COwherein the carbon based feedstock may be in the size range of 50 μm to150 μm, the thermal plasma reactor provides a temperature gradient andthe CO₂ is introduced into the reactor at a flow rate of 1.0 L/min to10.0 L/min.

In another embodiment, the present disclosure again relates to a processfor carbon dioxide conversion comprising supplying a thermal plasmareactor and introducing into the reactor a carbon based feedstock (C)and carbon dioxide (CO₂) wherein the thermal plasma reactor is at atemperature sufficient to convert the carbon based feedstock to anoutput gas comprising carbon monoxide according to the followingreaction:C+CO₂⇄2COwherein the carbon based feedstock is in the size range of 50 μm to 150μm, the thermal plasma reactor provides a temperature gradient and saidCO₂ is introduced into said reactor at a flow rate of 1.0 L/min to 10.0L/min. The output gases of the thermal plasma reactor may then beintroduced to a nonthermal plasma reactor wherein said nonthermal plasmareactor provides conversion of carbon dioxide to carbon monoxideaccording to the equation:CO₂+H₂→CO+H₂O

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description below may be better understood with referenceto the accompanying figures which are provided for illustrative purposesand are not to be considered as limiting any aspect of the invention.

FIG. 1 illustrates a process flow diagram illustrating the use of carbondioxide as an oxygen resource to convert CO₂ to CO.

FIG. 2 illustrates a preferred configuration for syngas production forthe process flow diagram provided in FIG. 1.

FIG. 3 illustrates the thermal plasma apparatus herein and two samplingports: Sampling Port A and Sampling Port B.

DETAILED DESCRIPTION

The present disclosure relates to a carbon dioxide emission reductionprocess which may utilize carbon dioxide (CO₂) as an oxygen resource inthe presence of a carbon based feedstock and convert carbon dioxide tocarbon monoxide (CO) in a plasma gasification process. The process mayutilize a thermal plasma reactor as the primary syngas (carbon monoxideand/or hydrogen) production. It may then optionally employ a nonthermalplasma reactor for further carbon dioxide conversion which may takeplace in the presence of other reactants.

Attention is directed to FIG. 1 which provides an initial process flowdiagram. As can be seen, a carbon based feedstock such as coal and CO₂may now be configured for introduction into a thermal plasma gasifier.Accordingly, the process herein may utilize a relatively wide range ofcarbon based feedstocks which may include coal, oils, petroleum coke,refinery residuals and/or wasters, hydrocarbon contaminated solids,biomass and agricultural wasters. With regards to the preferable use ofcoal sources, one may preferably employ lignite, sub-bituminous,bituminous, and/or anthracite type material. The carbon based feedstockis preferably introduced into the thermal plasma gasifier in particulateform, where the particulate may be in the size range of about 100 meshto 300 mesh (50 μm to 150 μm). In addition, one may preferably introducethe carbon based feedstock into the thermal plasma gasifier at a rate of5.0 g/min to 20 g/min and at all 1.0 g/min increments therein. Forexample, one may introduce the coal at a rate of 6.0 g/min, 7.0 g/min,8.0 g/min, 9.0 g/min, 10.0 g/min, up to the level of 20.0 g/min.

The CO₂ for introduction into the thermal plasma gasifier may beobtained from a variety of sources and may preferably be recycled CO₂that may become available from any downstream operation. The CO₂ may beintroduced on its own as the delivery gas and optionally in the presenceof other gases such as air and/or oxygen. The CO₂ may preferably accountfor 50% or more of the gas volume flow. Accordingly, it will beappreciated herein that the use of CO₂ as the carbon based feedstockdelivery gas now serves as an oxygen carrier for the ensuing conversionto CO by a reaction with the carbon feedstock (coal) in the thermalplasma gasifier by a reverse Boudouard reaction. The Boudourad reactionmay be understood as the redox reaction of a chemical equilibriummixture of CO and CO₂ at a given temperature. The reverse Boudouardreaction may now be configured to take place on the surface of thegasified carbon based feedstock within the thermal plasma gasifier andmay be represented as follows:C+CO₂⇄2COMore specifically, the reaction of CO₂ with carbon involves thedetaching of an oxygen atom from CO₂ at an active site on the surface ofthe gasified carbon. the reverse Boudouard reaction may be written asfollows:

In the above, the first step may be understood as the dissociation ofCO₂ at a carbon free active site (C_(fas)) releasing CO and forming anoxidized surface complex [C(O)]. In the second step the carbon-oxygensurface complex may subsequently produce CO and a new carbon free activesite. The first step is relatively slow compared with the second step,so the second step may be treated as an irreversible reaction.Accordingly, desorption of the carbon-oxygen surface complex is the ratelimiting step. A reaction rate equation that reflects this mechanism maybe written as follows:R=k ₁ p _(CO2)/(1+(k ₁ /k ₂)p _(CO2)+(k ₂ /k ₃)p _(CO))where p is the partial pressure for each component and k₁, k₂ and k₃ arethe rate constants for the reactions noted above.

With respect to the introduction of CO₂ to the thermal plasma gasifier,the CO₂ flow rate may be configured herein to preferably provide a CO₂concentration within the thermal plasma gasifier of 5.0 vol. % to 40vol. %. More preferably, the CO₂ concentration within the thermal plasmagasifier may be in the range of 20 vol. % to 30 vol. %. The CO₂ flowrate which may be utilized may be in the range of 1.0 L/min to 10.0L/min. It should also be noted that one may utilize the CO₂ as thedelivery gas optionally in conjunction with other gases, such as air oroxygen. In such manner, one may include with the CO₂ delivery gas airand/or oxygen, where as noted above, the CO₂ volume flow may be 50% orgreater and the air and/or oxygen (if utilized) account for anyremaining volume flow. Within such parameters, the air and/or oxygen mayoptionally be present at a flow rate of 0.1-2.5 L/min, more preferably0.5-1.5 L/min.

A more detailed illustration of the process described herein may befound in FIG. 2. Once again, the carbon source (preferably coal) alongwith CO₂ may be introduced into the thermal plasma combustion chamber. Aplasma gas may be separately introduced which may be sourced from inertgas and/or air. The inert gases may include argon and/or nitrogen. Suchplasma gases may preferably be configured to flow through electrodeswithin the spraying gun and exit at the tip of the nozzle. One may alsoseparately introduce water vapor and oxygen via separate transfer linesand into separate nozzles that are positioned within the spraying gun.The relatively high temperature plasma gas stream will then react withthe mixture of coal and reactant gases (air and/or oxygen) to produce asyngas (CO and H₂). It may therefore be appreciated that by adjustmentof the water vapor introduced, the amount of H₂ ultimately produced mayalso be conveniently regulated. Accordingly, the volume flow of watervapor herein may preferably be in the range of 0.1-30.0%.

A thermal plasma reaction zone may be identified in the thermal plasmacombustion chamber and will be related to the temperature and velocityfields of the plasma flow. The temperature of the center of the plasmamay be as high as 10,000° C. More specifically, the temperature withinthe thermal plasma combustion chamber may be a gradient, wherein thetemperature at the bottom of the chamber cools to about 200° C.Preferably, the temperature of the plasma may be in the range of 1000°C. to 1500° C. Such temperatures may be influenced by heat transfer fromthe plasma to the wall. During coal gasification herein, control of theenergy flow density of the plasma flow, via use of a confined flow mayoffer additional advantages. Such confined plasma flow may be achievedby the introduction of a quartz tube (see FIG. 2) to the thermal plasmacombustion chamber. Such confined flow has some of the following addedbenefits:

-   -   1) The flow is confined and does not spread radially and the        plasma region becomes relatively longer.    -   2) The carbon material, which is treated by the thermal plasma        flow, may now reside in the plasma for a relatively longer        period of time and the heat exchange from the plasma to the        carbon material may be more effective.    -   3) The plasma flow comes into contact with the wall of the        quartz tube and the wall interaction may affect the reaction.

Accordingly, to further increase the residence time of the coalparticles in the hot zone (the region of plasma formation), as noted aquartz column may preferably be placed below the plasma spray gun. Thisallows for a relatively more efficient process for the CO₂ conversion toCO and H₂ herein through the reaction of the coal in the thermal plasmacombustion chamber. One example of a quartz column that may be employedherein includes a quartz column that has an inner diameter of 4.0-6.0inches with a height of 25.0 to 35.0 inches.

It may also be noted that heat transfer from the plasma to coalparticles is one of the factors that will affect the extent ofconversion in the gasification reactor. In the thermal plasma gasifier,coal is preferably injected into thermal plasma with an angledtrajectory to the plasma axis. For example, the coal trajectory maypreferably be at an angle of 45-60 degrees with respect to the verticalaxis of the plasma, as illustrated by the down-pointing arrowspositioned under the spraying gun in FIG. 2.

Heat transfer from the hot plasma jet to cold coal particles isspatially dependent on the transport coefficients, e.g., viscosity (μ),and thermal conductivity (k), and may be characterized herein by thePrandtl number (Pr), which is a dimensionless number, defined as theratio of momentum diffusivity (ν) and thermal diffusivity (α):Pr=ν/α=C _(p) μ/kwhere C_(p) is specific heat (J/kg·K), μ is dynamic viscosity (Pa·s), kis thermal conductivity (W/m·K). Since k_(plamsa)<<k_(coal), there willbe a uniform coal particle temperature and gradients in the plasma ascoal particles are passing through the plasma, assuming there is nodecomposition for the coal particles. However in practice, some coaldecomposition is unavoidable.

Fourier's number (Fo) may b used herein for heat conductioncharacterization. Fo=αt/R², where t is coal particle heating time, and Ris coal particle radius. Accordingly, relatively small coal particlesherein will experience relatively improved heat transfer from theplasma.

Since some of the chemical reactions of coal gasification are reversibleat high temperature, to improve the yield of the products herein,removal of heat is preferred which maybe achieved via a “quenching” orcooling operation. To quench the reactions in the coal gasification,cooling water jackets may optionally be installed to cool the reactionzone or the thermal combustion chamber itself. Another benefit ofcooling is to prevent the chamber wall from overheating. In addition,some gases can be used to quench the reaction such as the inert gas oreven additional amounts of CO₂. Accordingly, unreacted CO₂ from coalgasification from a downstream operation or CO₂ from other sources maybe employed to assist in quenching. To achieve zero-CO₂ emissions, CO₂may be separated from the syngas produced herein and the coal reductionreactor effluent to re-feed to the plasma reactor and provide cooling.

As illustrated in FIG. 1 and FIG. 2, the output of the thermal plasmacombustion chamber may optionally be fed to a cyclone separator forremoval of ash and the gases may then be introduced to a nonthermalplasma reactor (NPR). The purpose of the NPR is to provide for furtherconversion of CO₂ to syn gas (CO) which may be achieved by a reversewater gas shift reaction:CO₂+H₂→CO+H₂O

The NPR herein may preferably generate a non-equilibrium relatively lowtemperature plasma through glow discharge. Accordingly the NPR herein isone which is not in thermodynamic equilibrium as the ion temperature isdifferent from the electron temperature. That is, only the electronsattain the energy level typically found in the components of a typicalthermal plasma. Therefore, the same type of reactions and oxidation thatmay normally take place at high temperature is possible at relativelylower temperatures. In the present disclosure, the voltage feed to theNPR is adjusted between 0-45 kV to provide a glow discharge.

The NPR herein may be preferably operated in the presence of water andan inert gas (e.g nitrogen) and CO may be produced due to the electronimpact dissociation of CO₂. The possible reactions are therefore asfollows:

(1) CO₂ dissociation in the presence of water. The CO₂ conversion inthis path is relatively small. During the pulse discharge, CO isproduced due to the electron (e) impact dissociation of CO₂. Thepossible reactions leading to CO and H₂ production are shown below,which may arise from the dissociation of CO₂ and water:H₂O+e

OH+H→O+H₂CO₂ +e→CO+OO+O→O₂

(2) Reaction of CO₂ with an excited N₂ species. In a nonthermal plasmareactor at 200° C., extra CO can also be produced from the reaction ofCO₂ with excited N₂ species, as observed in our experiments. The CO₂conversion rate is less than 1%.

(3) The reduction of CO₂ with H₂. The CO₂ conversion rate iscontemplated to be the highest for the identified three pathways. Morespecifically, experiments confirmed that 8.9% of CO₂ was converted to COin a nonthermal plasma reactor at 200° C. in presence of H₂ withoutcatalyst. The formation of CO is via the reverse water-gas shiftreaction which is mildly endothermic, ΔH°=41.2 kJ/mol. One possiblereason for relatively higher CO₂ conversion is the dissociation of H₂Ounder plasma condition shifts the reaction to the right hand side, whichfavors the CO₂ conversion.CO₂+H₂→CO+H₂O

The competitive reaction, formation of CH₄ is via the followingreaction:CO₂+4H₂→CH₄+2H₂O

This reaction is moderately exothermic, ΔH°=−165 kJ/mol. The CH₄ yieldis generally below 1%. Since there is no driving force for furtherreduction of CO to CH₄, the CO₂ conversion in this path is relativelylower. With the further conversion of CO₂ in the presence of hydrogen,the CO/H₂ ratios may be adjusted to suite specific utilization of syngasfor the downstream applications.

EXPERIMENTAL EXAMPLES

Test Setup for Coal Gasification

During the gasification tests, argon gas was used as the plasma gas(working gas), which was flowing through inside the plasma spray gun tocreate a plasma jet stream down-flow into the reaction chamber. Argonwas also used as a carrier gas for coal delivery. The reactant gas, airor CO₂, was fed though a separate nozzle along with the coal feed streaminto the plasma reactor. Reactions occur within the plasma flame andwithin the subsequent reaction zone below the plasma flame in the quartztube (4″ O.D.×29″ height). The total volume of the reaction chamber isapproximately 212 liters. Gaseous species and part of the coal ash arevented from the lower outlet of the chamber to a solids separatorcyclone, in which the coal ash was separated and discharged from thebottom of the cyclone. Another portion of coal ash and slag that isformed during the gasification and settled down to the bottom of thechamber. In other words, this settled ash did not make it into thecyclone. The gaseous product was analyzed using two GCs equipped withTCD and FID detectors, respectively. Analytical gas samples werecollected at fixed time intervals. FIG. 3 depicts the thermal plasmagasification setup and two sampling ports: Sampling Port A and SamplingPort B.

Coal gasification products were then collected from reactions withvarious types of coals with air as the additional oxygen source for thecomparison of reactions with CO₂ as the oxygen sources. The productionof CO and CO₂ components were evaluated.

Using Air as the Oxygen Sources in the Presence of Water

1. Lignite Coal

Thermal plasma gasification of lignite coal with air (1 L/min) and watervapor (0.15 L/min) was tested. The electric current for the plasma gunwas 500 A, and the electric voltage was 30 V. The air flow rate was 1L/min, and the plasma gas pressure was 75 psi. Before air, water, andcoal were introduced into the reactor, the thermal plasma was turned onto preheat the chamber so that the top chamber temperature reached 400°C., and the middle section (outside the quartz tube) was 250° C. Thencoal was delivered into the chamber. The average coal delivery rate was7.4 g/min during the 20-min test period.

Table 1 lists the analytical results for the gaseous samples taken fromSampling Port A. The highest H₂ concentration was 13.34% at 18-minreaction time and then decreased afterwards to 11.41%; at the same time,the highest CO concentration was 10.73%, and the CO₂ concentration wasalso at the highest point (0.50%).

TABLE 1 Analytical Results for Thermal Plasma Gasification of LigniteCoal with Air and Water at Sampling Port A Sampling time, min H₂, vol. %CO, vol. % CO₂, vol. % 0 0.00 0.23 0.17 2 0.00 0.65 0.27 4 0.00 0.750.43 6 1.68 0.77 0.38 10 8.23 5.89 0.38 12 7.98 4.69 0.36 14 4.13 2.920.33 16 8.33 6.47 0.44 18 13.34 10.73 0.50 20 11.41 7.74 0.33

2. Subbituminous Coal

Thermal plasma gasification of subbituminous coal with air and watervapor (3 vol. %) was also tested in this period. The electric currentfor the plasma gun was 500 A, and the electric voltage was 30 V. The airflow rate was 1 L/min, and the plasma gas pressure was 75 psi. Beforeair, water and coal were introduced into the reactor, the thermal plasmawas turned on to preheat the chamber so that the top chamber temperaturewas 400° C., and the middle section (outside the quartz tube) was 250°C.

Table 2 lists the analytical results for the gaseous samples taken fromSampling Port A. The highest H₂ concentration was 13.1% at 4-minreaction time and then decreased afterwards; at the same time, thehighest CO concentration was 10.9%, and the CO₂ concentration was alsoat the highest point (0.3%).

TABLE 2 Analytical Results for Thermal Plasma Gasification ofSubbituminous Coal with Air and Water at Sampling Port A Sampling time,min H₂, vol. % CO, vol. % CO₂, vol. % 0 0.00 ND 0.00 2 2.24 10.828 0.074 8.47 10.919 0.16 6 10.75 10.832 0.14 8 13.09 10.898 0.27 10 12.3210.842 0.13 12 12.36 10.888 0.13 14 10.08 10.838 0.12 16 9.59 10.9620.18 18 8.09 10.831 0.19 20 8.72 11.012 0.23

The temperature of the plasma gas may be controlled by the voltageapplied to the electrodes in the plasma gun. The syngas yield form thecoal gasification may be controlled by the temperature effects of theplasmas gas as demonstrated in the following experiments.

1. Subbituminous Coal in the Presence of Air and Water

Thermal plasma gasification of subbituminous coal with air wasconducted. The electric current for the plasma gun was 650 A, and theelectric voltage was 30 V. The air and CO₂ flow rate were both 1 L/min.Before air and coal were introduced into the reactor, the top section ofthe chamber temperature was 500° C., and it increased to 656° C. afterthe gasification proceeded for 20 minutes. The average coal deliveryrate was 9.7 g/min.

Table 3 shows the analytical results for samples taken at Sampling PortA. For subbituminous coal gasification with air but without CO₂, CO andH₂ were produced with no CO₂ in the product. The H₂ concentrationschanged between 15.37% and 10.40%; the CO concentration was at 11.59% at10-min reaction time, and then decreased to 9.36% at 20-min reactiontime. The fluctuation of the product gas concentrations might be fromthe unsteady coal delivery during the test.

TABLE 3 Analytical Results for Sampling Port A During Thermal PlasmaGasification of Subbituminous Coal with Air (1 L/min) Sample ID SampleTime (min) H₂ conc. % CO conc. % CO₂ conc. % A10 10 15.37 11.59 0 A12 1211.90 9.42 0 A14 14 11.73 9.71 0 A16 16 10.40 9.07 0 A18 18 12.64 11.200 A20 20 12.04 9.36 0Using CO₂ as the Oxygen SourcesBackground Data for the CO₂ Introduced to the Plasma Reactor without thePresence of Coal

CO₂ was used as a delivery gas to replace argon flowing into the coalhopper. The CO₂ cylinder pressure was 75 psig, and the CO₂ flow ratemeasured with a flow meter was 5 L/min. To measure the CO₂ input fromthe coal delivery hopper, the argon plasma gas flow rate was the same asthe coal gasification, and the gasification chamber was preheated to500° C. with thermal plasma at 650 A and 30 V. After the system reachedsteady state (10 minutes from the CO₂ flow), samples were taken fromSampling Port B, and the analytical data shows that the average CO₂concentration was 24.0 vol % as listed in Table 4.

TABLE 4 CO₂ Concentration Measured from Sampling Port B with CO₂Delivery Gas Sample Time (min) CO₂ concentration (vol %) 10 23.8 12 24.214 23.9 Average 24.0Coal Gasification Using CO₂ as an Oxygen Source

1) Lignite

To study the temperature effect, CO₂ effect, and oxidant (air or oxygen)effect on coal gasification, different gasification conditions weretested for lignite coal gasification: electric current (350 A or 650 A),argon or CO₂ as the coal delivery gas, air or oxygen as oxidant for coalcombustion.

A. Plasma at 350 A, with CO₂ as Delivery Gas

Thermal plasma gasification of lignite proceeded at 350 A, with CO₂ asdelivery gas, and Table 5 lists the analytical results for the gaseoussamples taken from Sampling Port B. The average H₂ concentration was5.80% at the 10-20 minutes reaction time, the average CO concentrationwas 18.44%, CO₂ was 12.74%, and acetylene was 0.53%. The average coaldelivery rate was 13.6 g/min.

TABLE 5 Analytical Results for Thermal Plasma Gasification of Lignite at350 A with CO₂ Delivery Gas Sampling Time (min) H₂, vol % CO, vol % CO₂,vol % Acetylene, vol % 10 5.85 18.32 12.63 0.52 12 4.76 17.81 13.38 0.4014 4.90 17.05 13.67 0.52 16 6.20 19.04 12.32 0.61 18 6.44 19.51 11.920.62 20 6.65 18.92 12.54 0.49 Average 5.80 18.44 12.74 0.53The CO₂ conversion rate calculation: 100×(24.0%-12.74%)/24.0%=46.92%

B. Plasma at 650 A, with CO₂ as Delivery Gas

Thermal plasma gasification of lignite proceeded at 650 A with CO₂ asthe delivery gas, without oxidant. Table 6 lists the analytical resultsfor the gaseous samples taken from Sampling Port B. The average H₂concentration was 13.22% at the 10-20 minutes reaction time, the averageCO concentration was 31.53%, CO₂ was 6.06%, and acetylene was 0.10%. Theaverage coal delivery rate was 16.3 g/min.

TABLE 6 Analytical Results for Thermal Plasma Gasification of Lignite at650 A, with CO₂ Delivery Gas Sampling Time (min) H₂, vol % CO, vol %CO₂, vol % Acetylene, vol % 10 12.46 31.46 6.62 0.13 12 12.64 31.62 6.560.10 14 12.74 31.50 6.27 0.09 16 13.97 32.04 5.81 0.10 18 13.44 30.995.81 0.10 22 14.98 31.50 5.31 0.10 24 12.32 31.63 6.06 Average 13.2231.53 6.06 0.10The CO₂ conversion rate was calculated as:100×(24.0%−6.42%)/24.0%=73.25%

C. Plasma at 650 A, with CO₂ as Delivery Gas and Additional Air at 1L/min. C.

Thermal plasma gasification of lignite proceeded at 650 A, with CO₂ asdelivery gas and 1 L/min air flow. Table 7 lists the analytical resultsfor the gaseous samples taken from Sampling Port B. The average H₂concentration was 10.29% at the 10-20 minutes reaction time, the averageCO concentration was 28.60%, CO₂ was 7.43%, and acetylene was 0.05%. Theaverage coal delivery rate was 12.7 g/min.

TABLE 7 Analytical results for thermal plasma gasification of lignite at650 A, with CO₂ as delivery gas and 1 L/min air flow Sampling Time (min)H₂, vol % CO, vol % CO₂, vol % Acetylene, vol % 10 10.85 30.01 6.46 0.0712 10.33 29.66 6.97 0.05 14 9.00 29.22 6.74 0.05 16 7.04 23.92 10.930.02 18 7.32 22.32 11.80 0.06 20 10.29 28.60 7.43 0.05The CO₂ conversion rate was calculated as:100×(24.0%−7.43%)/24.0%=69.04%

C. Plasma at 650 A, with CO₂ as Delivery Gas and Additional Air at 1L/min

Thermal plasma gasification of subbituminous coal at 350 A with CO₂ asdelivery gas was tested. Table 8 lists the analytical results for thegaseous samples taken from Sampling Port B. The average H₂ concentrationwas 5.67% at the 10-20 minutes reaction time, the average COconcentration was 20.01%, CO₂ was 11.48%, and acetylene was 0.26%. Theaverage coal delivery rate was 13.6 g/min.

TABLE 8 Analytical Results for Thermal Plasma Gasification ofSubbituminous Coal at 350 A, with CO₂ Delivery Gas Sampling Time (min)H₂, vol % CO, vol % CO₂, vol % Acetylene, vol % 10 5.95 20.97 11.37 0.3512 6.16 20.33 11.29 0.28 14 5.46 20.71 11.83 0.26 16 5.04 19.72 10.960.26 18 5.81 19.64 11.23 0.25 20 5.60 18.67 12.18 0.18 Average 5.6720.01 11.48 0.26The CO₂ conversion rate was calculated as:100×(24.0%−11.48%)/24.0%=52.17%

2) Subbituminous Coal

A. Plasma at 650 A, with CO₂ as Delivery Gas

Thermal plasma gasification of subbituminous coal at 650 A was testedwith CO₂ as delivery gas. Table 9 lists the analytical results for thegaseous samples taken from Sampling Port B. The average H₂ concentrationwas 14.33% at the 10-20 minutes reaction time, the average COconcentration was 30.43%, CO₂ was 5.46%, and acetylene was 0.12%. Theaverage coal delivery rate was 14.8 g/min.

TABLE 9 Analytical Results for Thermal Plasma Gasification ofSubbituminous Coal at 650 A, with CO₂ Delivery gas Sampling Time (min)H₂, vol % CO, vol % CO₂, vol % Acetylene, vol % 10 15.23 31.33 5.18 0.1612 14.53 31.18 5.47 0.11 14 10.50 29.56 6.40 0.06 16 14.84 30.03 5.590.14 18 15.54 30.80 4.99 0.13 20 15.33 29.67 5.16 0.13 Average 14.3330.43 5.46 0.12The CO₂ conversion rate was calculated as:100×(24.0%-5.46%)/24.0%=77.25%

B. Plasma at 650 A, with CO₂ as Delivery Gas and Additional Air at 1L/min and 1 L/min Oxygen Flow

Thermal plasma gasification of subbituminous coal at 650 A, with CO₂ asdelivery gas and 1 L/min oxygen flow was tested. Table 10 lists theanalytical results for the gaseous samples taken from Sampling Port B.The average H₂ concentration was 11.13% at the 10-20 minutes reactiontime, the average CO concentration was 29.76%, CO₂ was 6.92%, andacetylene was 0.04%. The average coal delivery rate was 13.8 g/min.

TABLE 10 Analytical Results for Thermal Plasma Gasification ofSubbituminous Coal at 650 A, with CO₂ Delivery Gas and 1 L/min oxygenflow Sampling Time (min) H₂, vol % CO, vol % CO₂, vol % Acetylene, vol %10 11.34 30.89 7.16 0.04 12 9.28 28.20 7.72 0.03 14 10.29 27.90 6.490.03 16 10.96 30.05 7.18 0.03 18 11.76 30.70 6.67 0.04 20 13.16 30.846.26 0.04 Average 11.13 29.76 6.92 0.04The CO₂ conversion rate was calculated as:100×(24.0%-6.92%)/24.0%=71.17%Nonthermal Plasma for Additional CO₂ Conversion to Syngas

The nonthermal plasma reactor used herein was capable of generating anonequilibrium, low-temperature plasma through glow discharge. Voltageis adjustable between 0-45 kV. It was found that voltage influenced theperformance of the NTP reactor. At higher operating voltage, the glowdischarge is relatively much stronger; however, some sparks wereobserved at 16.2 kV, and this indicates that the plasma generation wasnot stable.

Reaction of CO₂ (2 vol. %)+H₂O (1 vol. %)+N₂ (Dilute Gas) Stream in aNonthermal Plasma Reactor

In the experiment, 2 vol. % CO₂ and 1 vol. % water were fed into the NTPreactor with N₂ as dilute gas. At 15.3 kV, CO was produced in 200 ppmfrom FT-IR analysis. The CO₂ conversion was about 1 mol %.

Reaction of H₂ (30 vol %)+CO₂ (2 vol %)+N₂ (Dilute Gas) Stream in aNonthermal Plasma Reactor

During the NTP test of reaction of CO₂ with H₂, the volume % for CO₂ andH₂ were also 30 vol. % and 2 vol. %, respectively. The benefit of CO₂conversion is to reduce the unreacted CO₂ from thermal plasmagasification, and to change the product distribution, because the CO/H₂ratio will be higher. During the test, the preheater temperature was550° C., and the NTP reactor temperature was 200° C. It was observedthat without plasma, CH₄ was produced at 5 ppm, and CO was produced at165 ppm. The formation of CO is via the reverse water-gas shift reactionwhich typically operates close to equilibrium, and is mildlyendothermic, ΔH°=41.2 kJ/mol.

Without plasma, this reaction is temperature dependent. At 450° C., theCO concentration was only 20 ppm; at 500° C., the CO concentration wasabout 80 ppm; at 550° C., the CO concentration was about 165 ppm (0.85mol % CO₂ conversion). The stoichiometric reverse water-gas shiftreaction which typically operates close to equilibrium, and is mildlyendothermic, ΔH°=41.2 kJ/mol. The formation of CH₄ is via the followingreactionCO₂+4H₂→CH₄+2H₂O

This reaction is moderately exothermic, ΔH°=−165 kJ/mol. Withoutcatalyst, the CH₄ yield was relatively much lower than that of CO.

After the nonthermal plasma was turned on, the CO production yieldincreased significantly, and the CO yield was affected by the voltage ofthe NTP reactor. When the NTP reactor voltage increased gradually from4.5 kV to 7.5 kV, 12 kV, and 15.3 kV, the CO concentration in thereactor outlet increased from 625 ppm to 780 ppm, 840 ppm, and 1780 ppm.After the NTP reactor voltage increased from 15.3 kV to 16.2 kV, the COconcentration dropped quickly to 630 ppm, and then increased to 1080 ppmand stabilized at this concentration. This shows that 15.3 kV was thebest operating voltage. At 16.2 kV, the quick drop of CO concentrationwas related to the unstable plasma discharge.

CO₂ conversion calculation: at 15.3 kV, the highest CO concentration wasobtained. CO was produced in 1780 ppm from CO₂, and the initial CO₂concentration was 2%. The highest CO₂ conversion rate is calculated asfollows:

$\begin{matrix}{{{CO}_{2}\mspace{14mu}{conversion}} = {100 \times \frac{\left( {{CO}_{2}\mspace{14mu}{concentration}} \right)}{\left( {{initial}\mspace{14mu}{CO}_{2}\mspace{14mu}{concentration}} \right)}}} \\{= {{{\left( {1780 \times 10^{- 6}} \right)/2}\%} = {8.9\mspace{14mu}{mol}\mspace{14mu}\%}}}\end{matrix}$

The methane production rate is calculated as follows:

$\begin{matrix}{{{CH}_{4}\mspace{14mu}{conversion}} = {100 \times \frac{\left( {{CH}_{4}\mspace{14mu}{concentration}} \right)}{\left( {{initial}\mspace{14mu}{CO}_{2}\mspace{14mu}{concentration}} \right)}}} \\{= {{100 \times {\left( {120 \times 10^{- 6}} \right)/2}\%} = {0.6\mspace{14mu}{mol}\mspace{14mu}\%}}}\end{matrix}$

The production of CH₄ was also associated with the operating voltage ofthe NTP reactor. Interestingly, the trend is opposite from the trend forthe CO production. At the optimized operating voltage (15.3 kV), the CH₄concentration was the lowest. One possible reason is that CH₄ is mainlyproduced from thermal reaction both in the preheater and the NTPreactor, rather than from the plasma reaction.

Temperature effect. When the temperature of the NTP reactor decreasedfrom 200° C. to 100° C., at 15.3 kV, the amount of CO productiondecreased from 1780 ppm to 1340 ppm, and the amount of CH₄ productiondecreased sharply to 5 ppm. This strongly supported the assumption thatCH₄ was produced from the thermal reaction, instead of plasma reaction.

Mechanism of CO produced from CO₂. The highest CO concentration obtainedwas 1780 ppm at 15.3 kV and 200° C. CO was generated either by directelectronic dissociation of CO₂ molecule from its ground state or fromthe anti-symmetric vibrational state of CO₂, which is an intermediarystate of CO₂ dissociation as described in the following equation:CO₂(Σ_(u) ⁺)→CO(X¹Σ⁺)+O

The hydrogen atom will react with O in the above equation to facilitatethe production of CO. The formation of O via the above reactions is thecontrol step at such input voltages. This is why the CO yield isrelatively much higher with the presence of hydrogen.

The benefit of CO₂ conversion is to reduce the unreacted CO₂ fromthermal plasma gasification, and to change the product distribution,because the CO/H₂ ratio is higher.

The results from the thermal plasma and NTP tests may be summarized asfollows:

1. Thermal Plasma Tests

Thermal plasma gasification of lignite, subbituminous and bituminouscoals with CO₂ as the delivery gas, air and/or O₂ was conducted at athermal plasma flame of 650 A and 350 A. GC analyses showed that H₂, CO,and CO₂ were present in the gaseous products.

-   -   1) At 650 A, with the CO₂ as coal delivery gas, the highest CO₂        conversion rate for lignite, subbituminous, and bituminous coal        was 73%, 77%, and 88%, respectively.    -   2) CO₂ conversion rate increased with the thermal plasma        electric current.    -   3) Water vapor in the feed increased the H₂ concentration in the        product.

2. NTP Tests:

-   -   1) NTP effect on the water-gas shift reaction (CO+H₂O→CO₂+H₂):        The highest conversion rate for the reaction of CO with H₂O was        0.34 mol % at 200° C. This implied that there was no significant        change for CO and water.    -   2) The reaction of CO₂ with water and diluent gas N₂: The CO₂        conversion extent was ˜1 mol %.    -   3) The reaction of CO₂ with hydrogen and diluent gas N₂: The        highest conversion rate for CO₂ was 8.9 mol %, and 0.6 mol % of        CH₄ was produced. It was also observed that, without NTP, the        reaction of CO₂ and H₂ was extremely temperature dependent, and        0.85% of CO₂ conversion was obtained at 550° C. in the        preheater.    -   4) The reaction CO with H₂: No significant reaction was observed        for the reaction of CO with H₂ under NTP condition.    -   5) The reaction of CO₂ and water vapor with coal tar model        compound indene under NTP condition. No indene was detected in        the outlet of the NTP reactor, and no significant amount of CO        was produced. Indene might react with the hydroxyl radicals        generated from water. The observed products were not identified.

The foregoing description of several embodiments of the invention hasbeen presented for purposes of illustration. It is not intended to beexhaustive or to limit the invention to the precise steps and/or formsdisclosed, and obviously many modifications and variations are possiblein light of the above teaching.

What is claimed is:
 1. A process for carbon dioxide conversioncomprising: supplying a thermal plasma reactor; introducing into saidreactor a carbon based feedstock (C) and carbon dioxide (CO₂) whereinthe thermal plasma reactor is at a temperature sufficient to convert thecarbon based feedstock to carbon monoxide according to the followingreaction:C+CO₂Σ2CO wherein: said carbon based feedstock is in the size range of50 μm to 150 μm, said thermal plasma reactor provides a temperaturegradient and said CO₂ is introduced into said reactor at a flow rate of1.0 L/min to 10.0 L/min; and said thermal plasma reactor generatesoutput gases comprising CO and said output gases are introduced to anonthermal plasma reactor.
 2. The process of claim 1 wherein saidtemperature in said thermal plasma reactor is present as a gradient andis in the range of 200° C. to 10,000° C.
 3. The process of claim 1wherein water and air are introduced into said thermal plasma reactorand hydrogen (H₂) is produced in said thermal plasma reactor.
 4. Theprocess of claim 3 wherein the water is introduced into said thermalplasma reactor at a volume flow rate of 0.1-30.0%.
 5. The process ofclaim 1 wherein said thermal plasma reactor includes a spraying gunwhich directs the plasma to define a plasma axis direction and saidcarbon based feedstock is introduced at an angle of 45-60 degrees withrespect to said plasma axis direction.
 6. The process of claim 1 whereinsaid carbon based feedstock comprises coal.
 7. The process of claim 6wherein said coal is one of lignite, subbituminous, bituminous, oranthracite coal.
 8. The process of claim 1 wherein said carbon basedfeedstock is introduced into said thermal plasma reactor at a rate of5.0 g/minute to 20 g/minute.
 9. The process of claim 1 wherein saidnonthermal plasma reactor provides conversion of carbon dioxide tocarbon monoxide according to the equation:CO₂+H₂→CO+H₂O.
 10. A process for carbon dioxide conversion comprising:supplying a thermal plasma reactor; introducing into said reactor acarbon based feedstock (C) and carbon dioxide (CO₂) wherein the thermalplasma reactor is at a temperature sufficient to convert the carbonbased feedstock to an output gas comprising carbon monoxide according tothe following reaction:C+CO₂Σ2CO wherein said carbon based feedstock is in the size range of 50μm to 150 μm, said thermal plasma reactor provides a temperaturegradient and said CO₂ is introduced into said reactor at a flow rate of1.0 L/min to 10.0 L/min; wherein said output gases are introduced to anonthermal plasma reactor wherein said nonthermal plasma reactorprovides conversion of carbon dioxide to carbon monoxide according tothe equation:CO₂+H₂→CO+H₂O; and wherein said thermal plasma reactor includes a quartzcolumn.
 11. The process of claim 10 wherein said temperature in saidthermal plasma reactor is present as a gradient and is in the range of200° C. to 10,000° C.
 12. The process of claim 10 wherein water and airare introduced into said thermal plasma reactor and hydrogen (H₂) isproduced in said thermal plasma reactor.
 13. The process of claim 12wherein the water is introduced into said thermal plasma reactor at avolume flow rate of 0.1-30.0%.
 14. The process of claim 10 wherein saidthermal plasma reactor includes a spraying gun which directs the plasmato define a plasma axis direction and said carbon based feedstock isintroduced at an angle of 45-60 degrees with respect to said plasma axisdirection.
 15. The process of claim 10 wherein said carbon basedfeedstock comprises coal.
 16. The process of claim 15 wherein said coalis one of lignite, subbituminous, bituminous, or anthracite coal. 17.The process of claim 10 wherein said carbon based feedstock isintroduced into said thermal plasma reactor at a rate of 5.0 g/minute to20 g/minute.